In a conventional automotive drive train, power is distributed from a centrally located power plant or engine to one or more drive wheels which propel the vehicle in a desired direction. Early automotive drive trains and some modern drive trains, such as for farm and certain industrial uses, deliver power to a single rear drive wheel though a simple clutch and chain-drive system. While this design provides a relatively simple and inexpensive drive train, it suffers from several well-noted drawbacks which make it unsuited for general vehicular use.
The most salient drawback is the limited drive traction or acceleration force available from a single driving wheel. Those skilled in the art will readily appreciate that the maximum acceleration force exerted on a vehicle is limited by the traction force or friction force produced by the driving wheel(s) in contact with the road surface. Once the static friction of the driving wheel on the road surface is overcome, the wheel begins to slip and any further power delivered to the wheel is dissipated in the form of heat energy rather than as an acceleration force exerted on the vehicle. If the drive wheel gets stuck in mud or ice, the vehicle becomes disabled. For a multi-wheeled vehicle, therefore, it is desirable to deliver power to as many wheels as possible in order to maximize traction and vehicle acceleration.
A more subtle drawback is the imbalance of torque created by the exertion of a driving force on one rear wheel, but not the other. As power is delivered to the drive wheel during vehicle acceleration, an imbalance is created which causes the vehicle to veer or swerve in a direction away from the driven wheel. Conversely, when the drive wheel imparts a deceleration force the vehicle will have a tendency to veer or swerve toward the driven wheel. This causes undesirable vehicle handling performance.
To balance the acceleration and deceleration forces imparted to a conventional four-wheeled vehicle, it is desirable to distribute driving power over at least the front two wheels (front-wheel drive) or rear two wheels (rear-wheel drive) of a vehicle. This balances the acceleration and deceleration forces exerted on the vehicle and also increases drive traction. For off-highway driving or driving on wet or icy pavement it may be desirable to distribute power to all four wheels (four-wheel drive) in order to provide maximum traction under these driving conditions.
Modern vehicular gear trains provide balanced power distribution via a differential gear assembly disposed between the left and right axles of a driven pair of wheels (font and/or rear). A conventional automotive differential consists of a pair of opposed beveled side gears secured to the inboard side of each half-axle and engaging a centrally disposed pair of pinion gears mounted on a common pinion shaft. The pinion shaft is rotated about its transverse axis so as to apply equal forces to each side gear, delivering balanced power to the drive wheels. During vehicle cornering or turning the pinion gears allow the side gears to rotate relative to one another or "differentiate" so as to accommodate a relatively higher rotational speed of the outer drive wheel and a relatively lower rotational speed of the inner drive wheel.
Most modern vehicle drive trains utilize a conventional "open" differential. An open differential always divides torque equally between the opposing drive wheels. This provides optimal power delivery to the wheels under most driving conditions. However, if one wheel loses traction and starts spinning with only a small amount of torque applied, the other wheel also receives only this same small amount of torque such that the vehicle could easily become disabled. This problem is particularly acute when driving in muddy off-highway conditions or on wet or icy pavement. While an open differential divides torque equally between the drive wheels, maximum available torque is determined by the wheel having the least resistance to turning, which is undesirable.
Designers of certain high-performance racing vehicles have long attempted to overcome this problem by providing a "solid" rear axle such that both rear wheels are coupled together and driven in unison. A solid axle allows the torque on one wheel to be maintained regardless of the level of torque exerted on the other wheel. Unlike an open differential, a solid axle has the desirable and advantageous characteristic that maximum available torque is determined by the drive wheel having the most traction such that adequate driving traction can be maintained even if one wheel slips.
While solid axles are highly desirable under certain driving conditions such as for off-highway or high-performance automotive racing where a high degree of traction is required, they can present several undesirable drawbacks under most normal driving conditions. In particular, a solid axle provides no differentiation between the outer and inner drive wheels during cornering. This can cause, among other things, severe under-steering of the vehicle, undesirable scuffing of the tires and uneven tire wear as the wheels are forced to maintain uniform rolling speed even during cornering maneuvers. A solid axle can also strain the vehicle suspension system since the axle will resist turning during cornering. These drawbacks make solid axles generally unsuited for most vehicular uses.
Many attempts have been made to design a hybrid vehicular differential which combines the traction enhancing advantages of a solid axle with the balanced power delivery and differentiating capability of a conventional open differential. Two basic types of hybrid differentials have been proposed--"limited slip" differentials and "locking" differentials. Limited slip differentials generally utilize a friction plate or slip plate to transmit a portion of the torque from a slipping wheel to a non-slipping wheel. Limited slip differentials do not provide the full traction power attained using a solid axle, however, because only a portion of the available torque can be transmitted to the non-slipping wheel while still allowing for adequate differentiation under normal driving conditions. Also, limited slip differentials are not 100% energy efficient since a portion of the available power is typically dissipated as heat energy in the friction plate.
Locking differentials, on the other hand, utilize a releasable locking mechanism to deliver 100% power to both wheels during straight-away driving, but release one wheel during cornering maneuvers so that a differential function is achieved. See, for instance my U.S. Pat. No. 5,413,015, incorporated herein by reference. Locking differentials provide significant traction and performance advantages over conventional solid axle or open differentials. One particularly popular locking differential product is available from PowerTrax.TM. of Costa Mesa, Calif. under the trademark Lock-Right.TM..
The Lock-Right locking differential consists of two bidirectional clutches which replace the pinion gears and side gears of a conventional open differential. Each clutch has a driving member or "driver" and a driven member or "coupler." The driver mates with its coupler to form a fully locking clutch combination. When the vehicle is moving straight ahead, both wheels rotate at the same speed and both clutches are fully engaged. On the other hand, when the vehicle begins to turn, the outside wheel starts to overrun the inside wheel. This causes the outside clutch to ratchet, allowing the wheel to rotate freely as power is diverted to the slower moving inside wheel. As the vehicle straightens out, the wheels again rotate at the same speed and the outside clutch re-engages. This differentiating action occurs automatically for right and left turns and in both forward and reverse directions.
While such locking differential products have been well received by off-highway and high-performance automotive enthusiasts, original equipment manufacturers ("OEMs") have been slow to accept such products for use in new vehicles. OEMs have expressed several concerns with existing locking differentials. For example, the functioning of most locking differentials produces a series of clicking, ratcheting or clanking sounds during vehicle cornering as the clutch driver alternately engages and disengages the coupler. While these sounds are usually not a problem for off-highway or high-performance racing, they may be objectionable for day-to-day driving.
Existing locking differentials can also create an under-steering condition during vehicle turning as power is diverted from the outside wheel to the inside wheel. Again, while this behavior is generally not a problem for off-highway or high-performance driving, it may be objectionable for day-to-day driving where a more balanced power distribution would be preferred. OEMs are also concerned that frequent ratcheting of the driver and coupler may create increased wear and tear on the differential gear assembly leading to decreased durability.
Several existing locking differential manufacturers have attempted to address some of these concerns by adding compensating components, such as hold-out rings, additional pinion gears, and silencers. However, these modifications do not eliminate occasional clanking sounds, and add significant numbers of components representing substantial increase in both labor and material costs. Other manufacturers offer locking differentials which are selectable, such that they can be engaged or disengaged, as desired. However, these products have many external components and are not adapted to be fitted into a standard differential casing, making them prohibitively expensive either as aftermarket items or as OEM vehicle options.
For example, ARB of Victoria, Australia offers a selectable locking differential which replaces the entire differential gear assembly and differential casing of a vehicle. The ARB product utilizes a pneumatically-actuated piston to lock one of the side gears to the differential casing such that up to 100% of the torque is delivered to the other side gear through the pinion gears. Additional pinion gears and shafts are added to carry the increased torque. An air compressor is also required to be installed in the vehicle to produce the pressurized air needed to operate the ARB locking differential. For retrofit installations, a hole must be drilled through the differential carrier and axle housing in order to introduce a pneumatic control line. The installation procedure alone for installing the ARB locking assembly, pneumatic actuator and air compressor is prohibitively expensive for many automotive consumers and may, at least for retrofit applications, introduce abrasive metal particles into the differential gear assembly.